Multistage process of pulping raw cellulosic material



Patented Sept. 27, 1932 UNITED STATES PATENT OFFICE GEORGE A. RICHTER, OF BERLIN NEW HAMPSHIRE, ASSIGINOR TO BROWN 001mm, 01 BERLIN, NEW HAMI'SHIRE, A CORPORATION OF ,HAINE MULTISTAGE PROCESS OF PULPING RAW CELLULOSIO MATERIAL Ho Drawing.

This invention relatesto a multi-stage process of pulpin raw cellulose materials,-for instance, woo s such as spruce and fir. Its

object is the economical production of a pulp 5 having a more desirable composition and better physical characteristics, especially tear resistance, than pulps derived from the same raw material by the usual one-step chemical pulping process, such as the sulphite, soda, or kraft processes.

When solutions of sodium sulphite, especially those containing alkali, are used to libcrate pulp bythe cooking of raw cellulosic materials therein, it is found that the pulps derived from woods such as spruce are comparable in their composition to the usual kraft pulps. That is to say, the pulps are not very high in alpha cellulose content and are characterized by a relatively high proportion of pentosans.

I have found that it is highly desirable to subject the raw cellulosic material to an initial cooking or extracting operation in substantially only water before its pulping is accomplished in a sulphite cooking liquor more particularly of the non-acid variety. When the initial cooking or extracting step is carried out under the appropriate temperature conditions, I have observed that a notable increase in the alpha cellulose content and a decrease in the pentosan content of the resulting pulp are effected, coupled with a marked appreciation of its tear resistance. From the standpoint of securing the best results, the initial step should be performed in water at or above its boilirag point, preferably between 125 to 175 since lower'temperatures evidently do not favor ,much extraction of undesirable constituents, such as pentosans, from the wood, whereas higher temperatures evident-1y promote such extraction, but at the same time cause a transformation of the ligneous content of the wood into reaction products difiicultly soluble during the subsequent cook to complete fiber liberation, The transformation of the ligneous content ofthe wood ma be caused by the organic acids generated rom the pentosans, for I have noted a similar transformation with acid solutions, e. g., acid Application med February 18, 1931. Serial No. 518,848.

sulhite solutions, in which latter case, too, di culty is experienced in liberatin fiber from the acid-cooked wood in non-aci sodi= um sulphite liquors. If temperatures above C. are used, it is practically impossible to remove much of the lignin but even thou in such higher temperatures are employed in the first step, it ddes not prevent the pulping action of the liquor used in the second step. The resulting pulp, however, will be brow'n at and very high in lignin. There will be no reasonable wa in which the fiber can be bleached, but 1t could be used for urposes' which do not demand a white cellu ose. As an example of such a case, I can cook woodin water at, say, C. and remove a considerable proportion of the pentosans which are present in the original wood. The resulting mass is dark brown in color. When it is pulped with a sodium sulphite solution, I have found it necessa to use a higher temerature than ordinari y employed, and that may be compelled to go to temperatures as hi h as 390 or 400 F. The resulting pulp will e very low in pentosans, approximating M 1% or less. The lignin content may ran from 25% to 40%, which is conslderab y higher than the ligmn content of most pulps, including groundwood. The li 'n content of the resulting pulp, based on pu p, may even be higher than that of the original" wood, based on wood, because of a partial removal of material from the wood during the first and second steps in my pulping sequence. By practising the initial water cook between es the lower temperature limit at which little benefit accrues and the upper temperature limit at which subsequent fiber liberation is hindered, I can attain my objective, namely an improved pulp at low cost, for the init1 00 cook or extraction requires no chemicals and the second cook may be completed in a period of time no longer than usual and with no more chemical than in a single-step cook to complete fiber liberation.

The value of my invention can at once he recognized by comparing the results obtained by cooking raw wood, such as spruce in a straight or neutral sodium sulph te solution with those realized when the raw wood under- I went an initial extraction with water, as hereinbefore described, before a cook in a neutral sodium sulphite solution under conditions similar to those maintained in the single-step cooking operation. The use of a neutral sodium sulphite solution is of advantage, as it permits the two-stage cooking operation of the present invention to be carried out in a single digester, for instance, a steel digester, even though a slight acidity is developed in the water used in the first step. If this sodium sulphite solution, and digested for SIX hours at a temperature of about 180 C. to produce pulp. The yield of pulp was 44% based on the weight of wood used as a raw material. The washed pulp had an alpha cellulose content of 86% and a pentosan 'content of 7 .4%. When beaten accordin to the procedure followed in testing kraft pu ps and then formed into test sheets of waterleaf paper, the sheets were found to have an average pop-strength of 213 and an average tear resistance of 139. The two-stage procedure of the present invention was initiated with spruce chips taken from the same batch as in the example run for purposes of comparison. The chips were placed in a digester, drowned in water, and then cooked under confinement for four hours, at 150 C. At the end of this initial cooking operation, the chips were still hard and had practically the same integrity as the raw wood. They were drained and washed free of the aqueous cooking liquor, which was brown and feculent. The washed chips were then digested in a sodium sulphite solution under conditions,

duplicating those of the sin le-step comparative run. The yield of pulp was somewhat lower than the comparative run, namely, 37% based on the weight of wood used as a. raw material, but the washed pulp was much superior in composition, having an alpha cellulose content of 94.7% and a pentosan content of 3.6%. Test sheets made and tested in the same way as those prepared from the single-stage pulp had an average pop-strength of 185 and a tear resistance of 280. An enhancement of the important physical characteristics of tear resistance thus accompanied the betterment of the composition of the pulp. The slight deterioration of the strength of the pulp is of no consequence, as in actual paper and cellulose derivatives, and, as will be appreciated by those skilled in the art, commands a higher degree of utility in both these spheres of use than the usual commercial wood pulps.

The process of the present invention is not restricted to soft woods, such as spruce, fir, and poplar, but to hard woods, such as birch and maple, and to other raw cellulosic materials, such as bamboo, bagasse, or hemp. In carrying out the initial cookin step, neutral materials, such as sodium nitrate, zinc chloride, sugar, etc., may be added to the water to increase the ease with which pentosan groups are removed from the wood without injury to the cellulose group. The aque ous liquor from the initial cook may be separated from the chips and the chips washed, as described in the-foregoing example, but, if desired, the hot liquor associated with the chips may be retained in the digester and a sufiiciently concentrated solution of sodium sulphite and other desired chemicals may be added thereto to produce a secondary cooking liquor of the proper concentration. The secondary cooking liquor may contain onlysodium sulphide, as in the foregoing example. It may contain a subordinate proportion of alkali, such as sodium hydroxide, sodium sul-' phide, sodium carbonate, or even a sinall amount of sodium bisulphite, in which latter case the liquor will be slightly acid rather than alkaline. When the secondary chemical cookin liquor is produced in the digester from t e aqueous liquor resulting from the initial cook, the chemicals for such cooking liquor may be introduced into the di ester by injecting or pumping them as a highly concentrated solution into the closed digester so as to conserve the heat of the digester contents. Some space may be left, as ordinarilyi in the digester when it .is initially charge with the raw materials, including wood and water, to accommodate the conne trated chemical solution to be added there 0, although some water extract may be discharged from the digester for such urpose after the initial cook, if the digester 1s initially loaded with the raw materials substantiallyto its capacity. The chips may be partially or completely submerged in water during the initial cook. In fact, the initial cook may be conducted in an atmosphere of wet steam. In any case, volatile matters extracted from the wood may be intermittently released together with steam from the digester and, if de ired, recovered by condensation. The secondary cook should be carried out at a temperature ranging from about to 200 C. and for a period of from about three to ten hours, depending upon the composition and concentration of the secondary cooking liquor, the character of the raw cellulosic material used, and the modification which it has undergone during the initial cook.

I claim:

1. A process which comprises cooking raw cellulosic material under superatmospheric pressure in substantially only Water at a temperature above not less than 125 (1., and then cooking in a solution containing sodium sulphite as the main fiber-liberating agent to complete fiber liberation.

2. A process which comprises cooking is cod chips under superatmospheric pressure in substantially only water at a temperature of about 125 to C., and then cooking in a solution containing sodium sulphite as the main fiber-liberating agent to complete fiber liberation.

3. A process which comprises cooking raw cellulosic material under superatmospheric pressure in substantially only water at a temperature not less than 125 (3., and then cooking in a solution containing essentially only sodium sulphite to complete fiber liberation.

4. A process which comprises steaming raw cellulosic material under superatmospheric pressure at a temperature not less than 125 C., and then cooking in a substantially neutral solution of sodium sulphite to complete fiber liberation.

5. A process which comprises cooking raw cellulosic material under superatmospheric pressure in substantially only water at a tamperature not less than 125 C., and then cooking in a solution containing alkali and a preponderant proportion of sodium sulphite to complete fiber liberation.

6. A process which comprises cooking chipped wood under superatmospheric pressure for about four hours at about 150 C. in substantially only water, and then cooking for about six hours at about 180 C. in a solution of essentially only sodium sulphite to complete fiber liberation.

In testimony whereof I have afiixed my -sig nature.

GEORGE A. RICHTER. 

